The present invention relates to a fixed displacement gear pump of the type including seal bushings upon the outer ends of which act the pressures, and more particularly an improvement of hydraulic sealing arrangement wherein the side faces of a pair of intermeshing impeller gears are sealed by the bushings which rotatably support the shafts of the gears.
In the gear pumps of the type described above, the shafts of the intermeshing impeller gears are supported rotatably by the bushings which in turn are axially slidably fitted into the casing, and defined between the outer ends of the bushings on each side of the casing and an end plate are a low pressure chamber into which is admitted the low pressure at the suction opening and a high pressure chamber into which is admitted the high pressure at the discharge opening. In operation the high and low pressures act on the outer ends of the bushings to counteract the hydraulic pressures acting on the inner ends of the bushings in contact with the side faces of the gears so that the inner ends may be optimumly pressed against the side faces of the gears for hydraulic sealing.
As is very well known in the art, as the spaces between the teeth of the gears pass the suction opening, liquid is impounded in the spaces between the teeth, carried along the casing to the discharge opening and then forced through this opening. The passage of liquid through the gear pump may be divided into a low pressure sector in communication with the suction opening, a pressure transition sector wherein the pressure is gradually increased and a high pressure sector wherein the pressure is equal to the pressure in the discharge opening because the spaces between the teeth of the gears are communicated with the discharge opening through a clearance produced between the outer ends of the teeth and the casing when the gears are pressed toward the suction opening because of the pressure difference between the suction and discharge openings. The hydraulic pressure in the pressure transition sector changes over a wide range depending upon operational factors such as rotational speed of the gear pump, the temperature of liquid being pumped and so on so that the loading forces acting on the bushings for pressing them against the side faces of the gears are unbalanced and consequently operational noise is produced. In the worst case, the seizure of bushings occurs, resulting in serious damage.
In order to overcome these problems, there has been proposed a countermeasure wherein speed slots are provided to intercommunicate between the high pressure sector and the spaces between the teeth of the gears passing through a part of the pressure transition sector so that the pressure in the part of the transition sector may be substantially equal to the pressure in the high pressure sector. Furthermore, a second high pressure chamber or moderate pressure chamber is defined between the outer end of the bushing and the end plate at the position corresponding to a part of the pressure transition sector adjacent to the low pressure sector, and is communicated with the space between the teeth of the gear passing the transition zone adjacent to the low pressure sector.
By this arrangement, the bushings may be pressed against the side faces of the gears under almost uniform loading forces independently of the rotational speed of the gear pump and the temperature of liquid being handled, but there arises the problem that the speed slots cause errosion so that the service life of the gear pump is considerably reduced. The only solution to this problem is the elimination of speed slots. However, if speed slots are eliminated, the desired loading pressures act on the bushings only when the gear pump is driven at a speed within a limited range. At high or low speeds the loading forces are unbalanced so that leakages are increased with the resultant decrease in volumetric efficiency and excessive wear of rubbing surfaces of the bushings and gears occurs. It is considered that the unbalanced loading forces are caused by the improper selection of the area of the second high pressure or moderate pressure chamber.
So far O-rings have been used for defining the second high or moderate pressure chambers so that it is difficult to define a moderate pressure chamber having a sufficient area. Therefore the loading force in the moderate pressure chamber cannot follow the change in pressure in the transition sector when the gear pump is driven at high or low speeds so that the loading forces are unbalanced. Some attempts have been made to define the moderate pressure chambers without the use of O-rings, but so far they have not been successful in practice because parts are complex in construction, the steps for machining and assembly are increased in number and consequently the cost is increased.